Water Fleas, Hydras and Frog Clutches

A Pond Under the Microscope

1Every month from May to October, Ms. Odette Fairbanks’s sixth graders at Linwood Middle School collect water from Heron Pond behind the school and study it under microscopes. The school, the pond and the numbers here are made up, but they are based on real pond studies. This year, Kalani Mercer kept the class records.

2The most common animals in the samples are water fleas, or Daphnia: clear, shrimp-like animals about 1 to 3 mm long. Under the microscope, students could see eggs and young inside each female’s brood pouch on her back. In many kinds of Daphnia, a female in spring and summer makes eggs that develop without being fertilized. Her young get all of their genetic information from her, and almost all of them are female. When conditions get worse, such as colder water or shorter days, some young develop into males. Females then mate and make resting eggs, which are wrapped in a tough, dark case and can survive freezing. In June, Kalani put one female Daphnia alone in a jar of pond water. Three weeks later, the jar held 38 Daphnia, all female.

3The class also watched a hydra, a tiny animal with tentacles that clings to pond plants. A small bump grew on the side of its body, sprouted tentacles and broke off as a new hydra. A dish that started with 2 hydras held 12 hydras after 12 days.

4In the spring, a biologist from a nearby university, Dr. Tomás Ilunga, visited with data on a frog species his lab raises. In this species, skin color is controlled by one gene with two alleles. The allele D gives normal dark, spotted skin and is dominant. The allele d is recessive: a frog with two d alleles (dd) is albino, with pale yellow skin and pink eyes. Each tadpole gets one allele from its mother’s egg and one from its father’s sperm. His lab counted the tadpoles from three clutches of eggs, each from a different pair of parents.